This invention relates to a differential pressure, thermoforming machine and more particularly to a differential pressure, forming machine having new and novel apparatus for incrementally advancing a sheet of plastic through the machine.
Differential pressure forming machines, such as that disclosed in U.S. Pat. No. 3,496,257 granted to G. W. Brown, et al., on Feb. 17, 1970 and U.S. Pat. No. 3,664,791, granted to G. W. Brown on May 23, 1972, have pairs of laterally spaced, longitudinally extending chains for advancing sheets of plastic therethrough. These patents disclose a drive system for the sheet support chains comprising a cooperating drive rack and pinion gear, the rack being reciprocally driven by the piston of a fluid operated cylinder. A clutch is provided for coupling and decoupling the pinion gear to the sheet support chains so that the sheet support chains are incrementally forwardly indexed. Many problems are associated with the use of such gear racks. The prior art drive rack system is difficult to maintain and difficult to keep properly adjusted. Accordingly, it is an object of the present invention to provide a differential pressure thermoforming machine of the type described which includes a new and improved drive train for incrementally forwardly indexing the sheet support chains.
Trimming devices are commonly mounted in-line and downstream of a thermoforming mold to trim the formed part from the remainder of the sheet. It is important to index the sheet the same distance so that the in-line trimming apparatus will properly trim the part. The prior art gear rack incorporated relatively expensive and complicated damper mechanism for damping the inertia built up during the sheet advancing stroke. Such damping apparatus is difficult and expensive to maintain. Accordingly, it is an object of the present invention to provide a differential pressure forming machine including a sheet indexing system which is simpler, less expensive to construct and easier to maintain in adjustment.
It is another object of the present invention to provide a differential pressure thermoforming machine of the type described including a drive train which has improved stroke cushioning capabilities.
Yet another object of the present invention is to provide a differential pressure thermoforming machine of the type described including a new and improved drive train having new and improved apparatus for controlling the stroke length.
Another object of the present invention is to provide, in a differential pressure thermoforming machine, a new and improved drive train for the sheet advancing chains including a drive chain driven in a to-and-fro path of travel by the pistons of a pair of fluid operated cylinders coupled to opposite ends of the drive chain.
Still another object of the present invention is to provide a differential pressure forming machine including a pair of fluid operated oppositely operating drive cylinders, drive mechanisms one of which dampens the travel of the other.
Yet another object of the present invention is to provide a differential pressure forming machine of the type described including a pair of fluid driven cylinders having pistons for driving a drive chain in a to-and-fro path of travel and including mechanism for eliminating slack in the drive chain.